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Full text of "On the Solar and Lunar Variations of Magnetic Declination at Bombay.--Part I."

1868.] Mr. C. Chambers on Magnetic Declination at Bombay, 161 

II. '^ On the Solar and Lunar Variations of Magnetic Declination at 
Bombay/^ — Part I, By Charles Chambers^ Esq., Superin- 
tendent of the Colaba Observatory. Communicated by B, 
Stewart^ LL.D. Received Jane 30, 1868. 

(Abstract.) 

The hourly observations of magnetic declination at the Government Ob- 
servatory, Bombay, have extended over a period of nearly a quarter of a cen- 
tury, but the present discussion is confined to the observations made in the 
seven years 1859 to 1865. After describing the instrument with which the 
observations w^ere made and the method of reducing them, the writer ex- 
hibits, by means of Tables and curves, the. following results :— 

1st. The agreement of the diurnal variation of the aggregate of easterly 
disturbances when different separating values are adopted. 

2nd. The same for the aggregate of westerly disturbances. 

3rd. The diurnal variation of the aggregate of easterly disturbances, ex- 
ceeding V'4 in amount, in the period of seven years. 

4th. The same for westerly disturbances. 

5th. The disturbance-diurnal variation, or the excess at each hour of the 
aggregate of easterly over the aggregate of westerly disturbances. 

6th. The aggregates of easterly, westerly, and easterly and westerly dis- 
turbances, and the numbers of disturbed observations in each month of the 
year. 

7th. The aggregates of easterly, westerly, and easterly and westerly dis- 
turbances, and the numbers of disturbed observations, in each of the years 
1859 to ] 865, and in the period of seven years. 

8th. The solar- diurnal variation of declination in each month of the 
year, and for the whole year. 

9th. The excess of the diurnal variation of declination for each month 
over the mean diurnal variation for the year. 

10th. The mean diurnal variation of declination for the half-years April 
to September and October to March, in each of the years 1859 to 1865. 

1 1th. The semiannual inequality in the mean diurnal variation of decli- 
nation. 

12th. the mean diurnal variation of declination for each of the years 
1859 to 1865. 

13th. The calculated values of the coefficients A.^, B.^, Ag, B^, A3, and B3 
in the equation 

df, ==A^cosn-hB^smn~\-A^cos2n'^'B^sm2n'}-A^cosSn-\-'B^sm3n -f&c...* 
which expresses the mean diurnal variation of declination for each month 
of the year, for the whole year, and for different half-years. 

14th. The same for the half-years April to September and October to 
March, in each of the years 1859 to 1863. 

15th. The solar-diurnal inequaUty of dechnation, in the calculation of 
which all disturbances are included. 



162 Mr. C. Chambers on Magnetic Declination at Bombay, [Dec* 10, 

16th. The variation from year to 3^ear in the range of the mean diurnal 
variation of declination 

17th. The secular change and semiannual inequality of absolute decli- 
nation. 

The diurnal variations of disturbance, both easterly and westerly, are 
found to be of definite and systematic character, and to be comparable with 
the same variations for other places ; the annual variation is not very 
regular, but the progression in the amounts of disturbance in different 
years accords well (with exception as to the incomplete year 1861) with 
the known character of the decennial variation. The mean diurnal varia- 
tion of declination, as well as its semiannual inequality, is of the general 
character due to the latitude in which Bombay lies ; the progression from 
month to month in the annual variation of the diurnal variation is also di- 
stinctly marked in all months except July. A semiannual inequality is 
shown to exist in the diurnal variation of declination whose times of oppo- 
sition are the equinoxes. It is found that this inequality not only exists, 
but has the same general character at five widely separated stations in the 
northern magnetic hemisphere, and also, with some modifications as to cha- 
racter, at two stations having south magnetic latitude. Its special charac- 
teristics are :— • 

1st. That, as in the typical mean diurnal variation of declination, there 
is scarcely any change during the night hours, and that the main varia- 
tion occurs during half the day, in this case between 18 hours and 6 hours, 
local astronomical time. 

2nd. That the range of variation differs from about half a minute to 
nearly a minute of arc. 

3rd. That the hour of noon is that about which the deviations due to 
this variation pass through zero, and on each side of which the inflexions 
of the representative curve are inversely, but, in respect to north latitude 
stations, symmetrically disposed. 

4th. The turning-points are 21 hours and 3 hours, the former being a 
maximum, and the latter a minimum for north latitude stations from Ja- 
nuary to June, and for south latitude stations from July to December ; 
and vice versa, for north latitude stations from July to December, and 
for south latitude stations from January to June. The solar- diurnal ine- 
quality of declination, in the calculation of which all disturbances are in- 
cluded, differs at no hour of the day by more than 0'*061 from the mean 
diurnal variation, which is calculated after the rejection of all observations 
disturbed to the extent of more than l'*4. 

The range of the diurnal variation of declination in different years is 
shown to be subject to a periodical variation, whose times of maxima and 
minima approach nearly to those of the maxima and minima of the decen- 
nial period in the amount of yearly disturbance. 

The secular change of absolute declination is found for the years 1859 to 
1865 to be an annual increase of easterly declination of 3''017; the semi- 



1868.] On the Inequalities of Terrestrial Magnetism, 163 

annual inequality to be an excess of 0''227 of easterly declination in 
the months October to March over its value in the months April to 
September. 



III. ^^ On the Diurnal and Annual Inequalities of Terrestrial Magne- 
tism^ as deduced from observations made at the Royal Observa- 
tory^ Greenwich^ from 1858 to 1863 j being a continuation of a 
communication on the Diurnal Inequalities from 1841 to 1857, 
printed in the Philosophical Transactions, 1863. With a Note 
on the Luno-diurnal and other Lunar Inequalities, as deduced 
from observations extending from 1848 to 1863.''^ By George 
BiDDELL iliRY, Astronomer Eoyal. Eeceived July 27, 1868. 

(Abstract.) 

The author states that the instruments employed are precisely the same 
which were used in the second part of the former investigation, from 1848 
to 1857, mounted in the same place, and treated in the same manner. In 
describing the treatment of the photographic curves, he first gives the 
number of days which have been omitted in different years ; because the 
character of the observations or curves was too disturbed to permit the 
usual treatment of the observations, or the drawing by hand of a pencil 
curve that would fairly represent the general course of the curve. 

The greatest numbers of omitted days occur in the years 1846, 1847, 
1848; 1851, 1852, 1853, 1854; 1859, I860. As the estimate of the 
amount of irregularity has been made throughout by the same person, he 
considers that these years may be accepted as those in which the disturb- 
ances were the greatest. If they point to any cycle at all, it is one of 6 or 
6| years. These days being omitted, the ordinates of the pencilled curves 
on the other days were used as basis of all the following investigations. 
For the solar inequalities, they were treated by collecting the measures for 
every complete solar day, or for every solar hour bearing the same ordinal 
number, according as the annual or diurnal inequalities were the subject 
of inquiry ; but in all cases these quantities were next grouped by months, 
and the monthly means were taken. 

In the further treatment, the means of the monthly means of every 
complete day for all the months of the same name in the different years 
were taken and corrected for secular change ; the corrected numbers do 
not appear to indicate any sensible annual equation. Then the means of 
the monthly means of every solar hour for all the months of the same name 
in the different years were taken, giving the diurnal inequalities on the 
mean of years for the twelve separate months ; and these present, for the 
declination (north to west) and horizontal force, for the period 1858 to 
1863, sensibly the same differences between the summer months and the 
winter months as those for the period 1848 to 1857. For the vertical-force